Gwangju - Reigning champion Caeleb Dressel swam a sizzling 100m freestyle heat on Wednesday as fallout from the bust-up between China's Sun Yang and Briton Duncan Scott continued to swirl at the world championships in Gwangju.

The American, who racked up seven world titles two years ago in Budapest, clocked 47.32 to top the morning timesheets with Australian Kyle Chalmers, the gold medallist at the 2016 Rio Olympics, only 13th quickest going into semi-finals.

Dressel has two golds already this week with the tattooed pin-up targeting eight, but it was the vial-smashing allegations dogging Sun that continued to overshadow events in the water.

"Felt good, that's kind of how I wanted to swim it," said Dressel, who was ushered quickly past reporters before the subject could turn to doping.

Scott, who refused to shake hands with Sun on the podium or pose for photos on Tuesday after taking bronze behind the Chinese giant in the 200m final, received a formal warning from FINA for his actions - along with Sun for his angry response.

After finishing fifth in the 200m individual medley heats, Scott claimed he had no knowledge of the official rebuke, shrugging: "I've got no clue. I don't know as much as you guys right now - I've made my statement, sorry."

Scott's run-in with Sun followed Australian Mack Horton's podium snub after losing to Sun in the 400m at the weekend. Horton was also warned by FINA for bringing the sport into disrepute.

The swimmers were protesting after a leaked FINA doping panel report claimed Sun had smashed vials of blood with a hammer after being visited by testers last year.

Sun - branded a "drug cheat" by Horton at the 2016 Rio Olympics over a prior ban - accused the Australian of "disrespecting China" and he reacted furiously to Scott's snub on Tuesday.

The triple Olympic champion wagged his finger in Scott's face and raged: "You loser, I win!"

While Scott kept his counsel, British team-mate Alys Thomas told AFP: "To me he seems his normal self. As a team we all support him."

Sun has always maintained his three-month suspension in 2014 for a banned stimulant was an innocent mistake and the medication was for a heart condition, with Chinese officials believed to have failed to update their therapeutic use exemption list.

Several swimmers have backed Horton and Scott's stand, including Olympic champions Adam Peaty and Lilly King.

South African Chad le Clos, runner-up to Sun in the 200m final in Rio, said: "I want my gold medal back from Rio. It shouldn't be allowed to have people cheating in sports."

Sun was cleared by FINA to compete, prompting World Anti-Doping Agency (WADA) to appeal to the Lausanne-based Court of Arbitration for Sport, which is set to hear the case in September.

Australian Mitch Larkin said of Horton and Scott: "It's very courageous, I back them 100 percent."

Hungary's former Olympic silver medallist Laszlo Cseh set the pace in the men's 200m individual medley with a 1:57.79, with Japan's Daiya Seto and defending champion Chase Kalisz in hot pursuit.

American Hali Flickinger led the way in the heats of the women's 200m butterfly in 2:05.96 from Hungary's European champ Boglarka Kapas.

China's Fu Yuanhui, who won hearts in Rio for her straight-talking and bubbly nature, topped the times in the women's 50m backstroke in 27.70 seconds, 0.15 ahead of Brazilian Etiene Medeiros.